1. Field of the Invention
This invention relates to the field of spatial light modulators (SLM) and more particularly to a method for measuring the low spatial uniformity of a digital micromirror device (DMD) spatial light modulator.
2. Description of the Related Art
The automatic testing of spatial light modulators (SLM) typically involves the use of a machine vision system that includes an illumination source, optics, motion control, and appropriate test methods. These test systems exhibit a pronounced low spatial uniformity roll-off due to lighting and optics variations in the system, as well as variations in the device under test, as shown in FIG. 1. FIG. 1a shows both low frequency roll-off and high frequency defects across a portion of a horizontal line in a SLM, as identified in FIG. 1b. To locate the low spatial non-uniformities defects caused from smudges, scratches, and other defects on the surface of the mirrors and cover glass is the goal of such a test. The low frequency roll-off at the edges is typically caused by the optics and illumination source. Other high frequency, individual mirror related defects are not of interest in this test and are isolated and extracted from the test data. These machine vision test systems can be calibrated to measure and separate out the low spatial system non-uniformities from that of the SLM device under test, leaving only the low spatial reflectivity uniformity at the surface of the device as the results.
However, when the spatial light modulator under test is a deformable mirror device (DMD), an additional low spatial non-uniformity exists that is caused by the angle of the off-axis illuminating light across the device relative to the tilted mirrors. Although well established procedures for measuring the low spatial reflectivity for SLM device are known, the compensation for DMD mirror tilt angle variations are not addressed and can introduce significant errors in the test data if proper compensation is not made.
Thus there is a need for a low spatial uniformity test method(s) that includes the varying relation between the off-axis light source and the tilt angle of the DMD mirrors. The invention disclosed herein addresses this need in two embodiments.